ARM: imx_v6_v7_defconfig: enable fb

Enable CONFIG_FB which is nowadays required for CONFIG_DRM_PANEL_LVDS,
CONFIG_DRM_PANEL_SIMPLE, CONFIG_DRM_PANEL_SEIKO_43WVF1G,
CONFIG_FB_MODE_HELPERS, CONFIG_BACKLIGHT_PWM, CONFIG_BACKLIGHT_GPIO,
CONFIG_FRAMEBUFFER_CONSOLE, CONFIG_LOGO, CONFIG_FONTS, CONFIG_FONT_8x8
and CONFIG_FONT_8x16.

Fixes: f611b1e7

 ("drm: Avoid circular dependencies for CONFIG_FB")
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>